An international all-male crew took their first mock "Mars-walk" on Valentine's Day, kicking their unprecedented 500-day simulation of a mission to the Red Planet into a whole new gear.

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The excursion follows the crew's simulated Martian landing on Saturday (Feb. 12). It is the first of three planned Mars-walk simulations during an intensive 10 days of ground exploration. The expedition comes complete with spacesuit-clad astronauts and fake Martian dirt.

(AP Photo/Sergey Ponomarev)

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For the simulated landing, three members of the six-man crew of the mock Mars mission– an experiment called Mars500 – went through the motions of undocking a lander from their spaceship, plunging through the Martian atmosphere and touching down on the planet's surface – all without ever leaving Earth.

(AP Photo/Sergey Ponomarev)

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The Martian landing simulation is a major milestone for the Mars500 mission, which began about eight months ago. On June 3, six volunteers – two from Europe, one from China and three from Russia – locked themselves inside a set of connected, windowless modules in Moscow.

(AP Photo/Sergey Ponomarev)

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The idea is this: Simulate an entire manned mission to Mars, from beginning to end, to better understand the physical and psychological challenges astronauts will face on real deep space journeys. Russia's Institute of Biomedical Problems and the European Space Agency are conducting the experiment, which reportedly costs $15 million.

(AP Photo/Sergey Ponomarev)

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"The goal is, predict everything we can, prevent anything we can before the mission, detect anything that goes wrong in the mission, and intervene once we've detected it," said David Dinges, a psychology professor at the University of Pennsylvania, in an interview before the landing. Dinges is leading a study attached to the Mars500 project.